Brushless DC fans are a type of brushless DC motor assembly that are widely used for cooling a variety of electronics components and systems, including personal computers. Such fans are relatively inexpensive. The motors driving the fans require only a DC supply voltage and relatively simple control circuitry.
There is increasing desire for variable speed control of fans, since systems designers typically design for "worst case" conditions, and consequently design such fans to operate continuously at maximum speed. At maximum speed such fans are typically their noisiest, and their power consumption is also maximum. However, the systems being cooled, and their associated components, seldom operate at worst case, and, thus, the typical operation of brushless DC fans results in greater than necessary noise and power consumption.
Particular to the PC industry is the desire to control fan speed from the computer operating system based on measurement of critical temperatures within the PC, such as the processor die temperature. Controlling a fan from the operating system would provide the capability to lower fan speed under normal conditions and increase it when conditions occur which require higher cooling capacity. Further, control of fan speed using a digital to analog converter controlling a variable DC voltage would allow software or other digital control of the fan speed.
In addition, there is increased desire to actually monitor fan speed such that, e.g., an uncontrolled reduction in speed can trigger an alert to notify the system so appropriate action can be taken. To implement such fan speed monitoring requires a signal from the fan, typically called a tachometer signal, which requires an additional wire.
Presently, PC manufacturers have defined IC devices, which provide a means of controlling fan speed with various methods implemented external to the fan, while monitoring the tachometer signal with an RPM counter. In conjunction with these, it would be desirable to provide all the following criteria:
RPM control over 30% to 100% of range PA1 RPM control using a DAC controlled variable DC voltage PA1 RPM control method providing high power efficiency throughout RPM range PA1 Tachometer speed signal available throughout RPM range PA1 Minimal external components (PC system PCB) PA1 No more than three connections to fan
Solutions have been proposed to provide variable speed control for two-phase brushless motor assemblies such as fans, while limiting the number of wires connecting to such assemblies to three, a desirable cost saving objective. An example of such a prior art solution is disclosed in U.S. Pat. No. 4,656,553, entitled "Electronically Programmable Universal Brushless DC Fan with Integral Tracking and Locked Rotor Protection," which issued on Apr. 7, 1987, and was assigned to Comair Rotron Inc. The scheme disclosed in this patent, while maintaining three wires for connection to the motor, nonetheless requires an integrated circuit voltage regulator, e.g. an LM 317, included in the motor assembly, as well as an additional IC in the assembly to internally control the DC motor drive voltage that, in turn, controls the motor commutation. It also requires various additional circuit elements such as resistors, bipolar transistors and Zener diodes, to effect such speed control of the motor. Speed control is limited, accomplished by the selection of the value of a resistor which is connected between two of the external leads of the motor. In addition, this approach does not provide a fan speed tachometer signal, which, if added, would require a fourth wire.
Thus, this approach suffers from the disadvantages of low flexibility in the programmability of the speed of the fan and from the fact that the regulation of the motor speed is accomplished by varying the level of the DC voltage driving the fan motor windings. The arrangement operates with less efficiency than is desirable, and dissipates excessive heat from the linear voltage regulator. In addition, the size of the motor is limited, as the circuitry included in the fan motor assembly requires two ICs.
Another prior art approach is shown in FIG. 1. This approach to speed control of two-phase brushless motor assemblies involves adjusting the DC voltage to the motor, applied between the supply and ground wires. The third wire is then used for the tachometer's feedback signal. The system 10 shown in FIG. 1 consists of a two-phase brushless motor fan assembly 12 connected to a system printed circuit board ("PCB"). A control integrated circuit ("IC") 16 includes an RPM monitor 18. The RPM monitor 18 receives a tachometer ("TACH") signal on a first wire 20 from the fan 12. A digital to analog converter ("DAC") 22 on the control IC 16 provides a control voltage on line 24 to the "+" input of a differential operational amplifier 26. The level of the control voltage on line 24 corresponds to the speed at which the fan 12 is to run. The output of the operational amplifier 26 is provided on line 28 to the base of an NPN bipolar transistor 30. The collector of bipolar transistor 30 is connected to a power supply 32, while its emitter is connected to one end of a first resistor R1. The other end of resistor R1 is connected to one end of a second resistor R2, with the other end of resistor R2 being connected to ground 34. The common connection point of resistors R1 and R2 is connected by way of line 38 to the "-" input of operational amplifier 26, thus providing a feedback path so as to maintain the proper drive voltage on line 28 for bipolar transistor 30. This, in turn, provides the proper voltage on line 36 to motor fan assembly 12 under varying load conditions to line 36, such that the voltage on line 36 is equal to the DAC output voltage present on line 24 multiplied by the formula R2 ##EQU1##
Note that the prior art fan system 10 of FIG. 1 requires that active power drive components be placed on the system PCB. These additional components not only take up space on the printed circuit board which might otherwise be used for other components, the bipolar NPN transistor 30 also dissipate power in the form of heat that must be dealt with on the printed circuit board itself. Notice also that the motor supply voltage provided on line 36 varies in dependence on the desired speed set in DAC 22. This creates potential problems in that circuitry internal to the fan, which commutates drive to the motor phase windings and generates the tachometer signal, are powered from this same voltage source. Thus, lowering the voltage level on line 36 to reduce fan speed may cause these internal circuits to have inadequate operating voltage, leading to malfunction. In short, trying to control fan speed by adjusting an external DC supply voltage may place a lower limit on the RPM to which the fan can be controlled, due to voltage headroom requirements of the internal circuitry.
Thus, there is a desire for an approved apparatus for controlling with high power efficiency the speed of a two phase brushless motor in a functional assembly, while limiting the number of wires needed for connection to the assembly to three. It is desired to have such an arrangement having control over a wide range of motor speed. It is desired to provide such an arrangement while maintaining adequate voltage for other components in the assembly throughout the motor speed range. It is desired to provide such an arrangement while providing a tachometer motor speed signal available throughout the motor speed range. It is desired to have such an arrangement where fewer power drive components are required on the system printed circuit board to which the wires of the assembly are connected. Finally, it is desired to have such an arrangement requiring fewer components overall, including on the printed circuit board internal to the motor assembly.